In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a gas turbine mounted on the same shaft. The flow of combustion gas turns the gas turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
In the gas turbine, an annular, circumferentially extending stationary shroud surrounds the tips of the rotor blades. The stationary shroud confines the combustion gases to the gas flow path so that the combustion gas is utilized with maximum efficiency to turn the gas turbine. The clearance between the turbine blade tips and the stationary shroud is minimized to prevent the leakage of combustion gases around the tips of the turbine blades. The stationary shroud provides a rubbing surface for the tips of the turbine blades. The design intent is for the turbine blade tips to rub into the stationary shroud, with the contact acting in the manner of a seal. The clearance between the blade tips and the stationary shroud, and thence the amount of combustion gas that can bypass the turbine blades, is minimized, thereby ensuring maximum efficiency of the engine. The stationary shroud must be manufactured to and maintained at highly exacting tolerances in order to achieve this efficiency during extended service.
The gas path surface of the stationary shroud is exposed to abrasion by the rotating turbine blade tips and also to erosion, oxidation, and corrosion by the hot combustion gases. The base metal of the stationary shroud is typically not highly resistant to the environmental attack and abrasion, and therefore an environmentally resistant rub coating is applied on the gas path surface of the stationary shroud. Over a period of time as the engine operates, the surface of the environmentally resistant rub coating is worn away, and some of the base metal of the stationary shroud may also be damaged and/or removed. The result is that the dimensions of the stationary shroud are reduced below the required tolerances for efficient operation of the gas turbine engine. Alternatively stated, the annular radius of the inwardly facing surface of the stationary shroud gradually increases, so that an increasing amount of combustion gas leaks around the tips of the turbine blades and the operating efficiency is reduced. At some point, the stationary shroud is no longer operating acceptably and the operation of the gas turbine degrades below acceptable levels.
Because of the high cost of the stationary shroud materials, rather than dispose of the stationary shrouds, it is desirable to repair the stationary shrouds by restoring the stationary shrouds to their original dimensions in accordance with preselected tolerances as determined by the engine's size as well as to restore the corrosion resistant properties to the flow-path surfaces. In the past, this restoration has been accomplished by low pressure plasma spray (LPPS), thermally densified coatings (TDC), the high-velocity oxyfuel (HVOF) process, or activated diffusion healing (ADH). The first three approaches restore the stationary-shroud dimensions using the rub-resistant coating material but do not restore the structural strength of the underlying shroud base metal. The fourth approach repairs holes and cracks in the shroud base metal, prior to re-application of the rub-resistant coating material.
In the work leading to the present invention, the inventors have observed that these approaches achieve the desired restoration of the dimensions of the stationary shroud, but do not restore its mechanical performance. The stationary shroud no longer has its necessary mechanical properties, so that there is a risk of mechanical failure of the stationary shroud. There is needed an approach by which the mechanical properties as well as the dimensions of the coated stationary shroud are restored. The present invention fulfills this need, and further provides related advantages.